Three Volcanic Cycles of Yellowstone

Three extraordinarily large explosive eruptions in the past 2.1 million years
each created a giant caldera within or west of Yellowstone National Park with
the spread of enormous volumes of hot, fragmented volcanic rocks as pyroclastic
flows over vast areas within times as short as a few days or weeks. The
accumulated hot ash, pumice, and other rock fragments welded together from their
heat and the weight of overlying material to form extensive sheets of hard lava-
like rock. In some sections, these welded ash-flow tuffs are more than 400 m
thick! These ash-flow sheets—from oldest to youngest, the Huckleberry Ridge,
Mesa Falls, and Lava Creek Tuffs—account for more than half the material erupted
from Yellowstone. The enormous outpouring of magma, 280 to 2,450 km3 during each
explosive event, led to the collapse of magma-chamber roofs, causing the ground
above to subside by many hundreds of meters to form the calderas.

Huckleberry Ridge and Lava Creek ash-flow tuffs

At the top of Mount Everts east of Mammoth Hot Springs, a brownish-colored cliff
held an important clue about the number of caldera-forming eruptions at
Yellowstone. Click on the image to learn more.

A general sequence of events was repeated in the evolution of each of
Yellowstone's three volcanic cycles:

A broad area, larger than that which will become the caldera is slowly uplifted.
This uplift reflects the development and rise of large volumes of rhyolite to
form a magma chamber at shallow depths in the Earth's crust. Stretching of the
crust above the inflating magma chamber leads to concentric and radial
fracturing and faulting at the surface, typically accompanied by the extrusion
of lava flows from these fractures.

At a critical stage in the evolution of the magma chamber, enormous volumes of
the over-pressurized rhyolitemagma erupt explosively through the ring-fracture
zone created above the magma chamber during inflation and uplift, producing
extensive ash-flow sheets. As the eruptions partly empty the chamber of its
magma, the roof of the magma chamber collapses along the same ring fractures to
produce a large caldera.

Post collapse volcanism includes the extrusion of rhyolite lavas and smaller
explosive eruptions of pyroclastic flows within or adjacent to the caldera. In
the present-day Yellowstone caldera, lakes formed where streams draining into or
along the margin of the caldera were dammed by these thick intra-caldera
rhyolite flows, including Shoshone, Lewis, Heart, and Yellowstone Lakes. Shortly
following collapse, the caldera floor may be uplifted by hundreds of meters in a
process known as resurgent doming; this uplift reflects renewed pressure as
magma rises again into the magma chamber. Hydrothermal activity (such as hot
springs and geysers) occurs during all three stages but, in the third stage, it
becomes the dominant or only visible sign at the surface of magmatic activity
below.

Scientists infer that rhyolitelava flows as well as the caldera-forming ash-
flow tuffs were fed from shallow magma chambers filled by the melting of rocks
of the lower continental crust below Yellowstone. The heat needed to facilitate
the melting process was supplied by the repeated injections of basalt magma from
the mantle into the shallow crust. Click on images for a larger-sized image
and description.

Basaltlava flows, though subordinate in volume to rhyolites, have erupted
throughout the 2.3 million-year volcanic history of the Yellowstone area. The
magma feeding these eruptions originated in the upper part of Earth's mantle and
resided only briefly in the crust before erupting at the surface. Click on
images for a larger-sized image and description.

The long-term nature of volcanism in this part of North America suggests that
more eruptions will occur as the Yellowstone National Park continues to evolve.
The most recent series of eruptions, 160,000 to 70,000 years ago, extruded more
than 20 thick rhyolitelava flows and domes, most of them within the youngest
caldera. Other post-caldera lavas are basalts, erupted around the margins of the
rhyolitic calderas. Based on Yellowstone's history, the next eruptions are
likely to expel lavas, which might be either rhyolites or basalts, possibly
accompanied by moderate explosive activity. Far less likely would be another
enormous outpouring of material that could lead to a fourth caldera.